In accordance with the recent development of the information and communication industry, electronic devices are becoming smaller, lighter, thinner, and more portable. As a result, there is a growing demand for high energy densification of batteries used as power sources for these electronic devices. As lithium secondary batteries are best able to meet these demands, research thereon is being actively conducted.
Lithium secondary batteries include a cathode, an anode, an electrolyte providing a pathway for the movement of lithium ions between the cathode and the anode, and a separator. Electrical energy is generated by oxidation and reduction reactions when lithium ions are intercalated and de-intercalated in the cathode and anode.
Lithium secondary batteries have an average discharge voltage of about 3.6 to 3.7 V, presenting an advantage in that the discharge voltage thereof is higher than other alkaline batteries and nickel-cadmium batteries. In order to achieve such a high driving voltage, an electrolyte composition which is electrochemically stable at a charge-discharge voltage range of 0 to 4.2V is required.
At the time of initial charging of a lithium secondary battery, lithium ions generated from a cathode active material such as a lithium metal oxide, or the like, migrate to an anode active material such as a graphite-based material, or the like, and are intercalated between layers of the anode active material. Herein, since lithium is highly reactive, it reacts with an electrolyte and carbon the composing the anode active material on the surface of the anode active material (such as a graphite-based material), thereby resulting in the production of a compound such as Li2CO3, Li2O, or LiOH. These compounds form a solid electrolyte interface (SEI) film on the surface of the anode active material.
The SEI film acts as an ion tunnel and allows only lithium ions to pass through. Since the SEI film has the effect of an ion tunnel, an organic solvent molecule with a high molecular weight moving together with the lithium ions in the electrolyte is inserted between the layers of the anode active material to prevent the anode structure from being destroyed. Therefore, it is possible to prevent contact between the electrolyte and the anode active material, and thus degradation of the electrolyte does not occur and the amount of lithium ions in the electrolyte is reversibly maintained, thereby enabling the charge/discharge to be maintained stably.
In the related art, it is difficult to expect to achieve an improvement in the lifetime characteristics if lithium ion secondary batteries since an uneven SEI film is formed in the case of employing an electrolyte that does not contain an electrolyte additive or an electrolyte that contains an electrolyte additive with poor characteristics. Further, even when the electrolyte includes an electrolyte additive, if the amount of the electrolyte additive is not able to be adjusted to a required amount, problems have been encountered in which the electrolyte additive causes degradation of a cathode surface or an oxidation reaction of the electrolyte during high temperature or high voltage reactions, ultimately resulting in an increase of the irreversible capacity loss of the secondary battery, with deterioration of the lifetime characteristics.